Multiple nozzle air blast web drying



April 6, 1965 'r. A. GARDNER MULTIPLE NOZZLE AIR BLAST WEB DRYING 4Sheets-Sheet 1 Filed Jan. 4. 1961 MENTOR. 20m" 4. Gaza/are W! m M April1965 T. A. GARDNER A 3,176,412

MULTIPLE NOZZLE AIR BLAST WEB DRYING Filed Jan. 4, 1961 4 Sheets-Sheet 2n 5 INVENTOR.

M1, MA Q ATTORNEY! April 1965 T. A. GARDNER MULTIPLE NOZZLE AIR BLASTWEB DRYING 4 Sheets-Sheet 3 Filed Jan. 4, 1961 IN V EN TOR. Tim/w?! '9.Gaza/vex April 6, 1965 T. A. GARDNER 3,176,412

MULTIPLE NOZZLE AIR BLAST WEB DRYING Filed Jan. 4, 1961 4 Sheets-Sheet 4u. I E

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o .5 L0 1.5 2.0 2.5 3.0 IN VEN TOR. DI5TANcl EETWEEN NozzLES- mcnes TQ,q GHQDNEE A 7' 7' GENE Y5 United States Patent 3,176,412 MULTIPLENOZZLE Am BLAST WEB DRYlNG Thomas A. Gardner, 513 Clark St, Neenah, Wis.Filed Jan. 4, 1961, Ser. No. 80,610 8 Claims. (Cl. 34-422) Thisinvention relates to multiple nozzle air blast web drying and is acontinuation in part of application Serial No. 662,891, filed May 31,1957, now abandoned, and application Serial No. 743,437, filed June 20,1958, now abandoned. The invention seeks to provide increased dryingcapacity by operating at low Reynolds number values upon the laminarboundary layer of air and vapor in immediate proximity to the Web to bedried. (Any drying gas may be used, the term air being employedgenerically since air is the gas most commonly used for drying.) Theinvention has been tested successfully in the drying of webs ofnewly-formed paper, the drying of paper coatings, and the drying ofinks, dyes and the like on the surface.

While it is convenient to refer to the nozzles which create the jets, itshould be noted that it is the jets of dehydrating gas that do thedrying and are required to have the critical dimensions and spacing.Thus the figures given above may be stated in terms of the jetsthemselves. The jets should be spaced from each other not less thanthree-quarters of an inch nor more than two inches. They shouldoriginate at a distance of no more than three-quarters of an inch andpreferably one-eighth to three-eighths of an inch from the surface ofthe work to be dried. The jets should have a total cross section at thesaid distance of origination which amounts to one and one-half percentto five percent of the area acted upon, the preferred percentage areabeing two to four percent.

The invention provides increased drying capacity by operating on theboundary layer film in immediate proximity to the web being dried insuch a manner that its resistance to the flow of heat and mass isreduced.

The invention is based on critical relationships of nozzles includingspecifically the ratio of total orifices area to total work area;nozzles spacing from the work to be dried; and the spacing of thenozzles from each other. The distance between nozzles is particularlycritical. It should be not less than of an inch nor more than 2 inches,with peak performance at a space slightly in excess of one inch. Thepercentage of total area of nozzle orifice to the total heat transferarea of the surface acted upon should be not less than 1 /2 nor morethan 5 /2%, with a preferred range from 2 to 4%. The distance of thenozzle from the surface of the work is less critical but should bebetween /s" and /s" for best results. The dryer shows no improvementover previously known dryers if this spacing is increased beyond of aninch.

The invention further seeks to provide equal drying across the entirewidth of continuous webs used in industry. Only if drying is uniform isit possible to operate at high temperature. The large flow area in thepreferred plenum compared to the flow area of the orifices ensures thatthe pressure drop in the orifices will exceed by many times the pressurevariations in the plenum. Thus the flow through all orifices isextremely uniform. Uniform drying across wide machines is dilficult tomaintain. Performance of the machine herein disclosed is very superior.

The invention also provides more economical drying by the conservationof heat and power ordinarily used in drying.

I A major complaint made with respect to previously known dryers is themassive loss of air into the room in which the dryer operates. Onvirtually all dryers, 50%

more or less, of the discharge from the first nozzle and the last blowsout into the atmosphere after impinging on the surface, because of itshigh energy. Because of the multiplicity of small capacity nozzles usedin the present dryer, the percentage of air lost to the room becomesvery small. In the present dryer the air loss with nozzles 1 inch apartis only one third as great as the loss from a dryer of equal size andair capacity with nozzles 3 inches apart.

The matter of air loss is important. Such air carries away both heat andvapor. With small air loss the dryer may be operated at hightemperatures and humidities. High humidity operation requires less airto be heated to secure the same amount of drying. Thus the thermalefiiciency is high. High humidity also provides high concentration oflatent heat, which is easier to reclaim if desired. In the present dryerheat reclamation is economically feasible; formerly this has not beentrue. In the drying of costly solvents, high solvent humidity permitseasier reclamation of the solvent.

There is a further advantage of the present dryer in that itaccomplishes a given drying job in very much less space than is requiredby any previously known machine of comparable capacity. This not onlyreduces the demands on floor space in the building in which the dryer isused, but it reduces heat losses attributable to radiation andconvection.

Another advantage of using very small opacity nozzles with high jetenergy is to provide the beneficial effect of high velocity flow withouthigh kinetic energy such as might disrupt fragile webs or unstablesurface coatings.

In practice, the web may be supported either by a relatively flat plateor by a drum or floated between opposed sets of drying nozzles. Thespeed of the web is broadly immaterial since the flow of drying air ispreferably at a rate so high as to make conventional rates of webmovement immaterial.

It is conventional to use heat in the drum or plate over which the webis trained for drying purposes. In the use of the present invention,evaporation of moistures or other evaporized liquids will occur sorapidly from the surface of the web as to make it possible to increasethe amount of heat used in the drum, at the same time eliminating byevaporative cooling the difliculties experienced in some dryers when theheat is excessive for the particular materials involved.

Laterally elongated non-circular nozzles preferably extending the fullwidth of the web are so organized that the jets of drying gases issuingfrom the nozzles are directed against the web substantially at rightangles to its surface or at an angle which, allowing for web movement,results in a pattern of flow and pressure gradient which issubstantially symmetrical from the stagnation point opposite eachnozzle. Each nozzle may provide a substantially continuous opening whichis as long as the width of the web, broken only by spacers whichmaintain its side margins at uniform distances.

A feature of the present invention is the provision of means for theescape from the vicinity of the drying surface of air which has beenspent in impinging on the surfaces, the escape being accomplishedwithout impairing the uniform drying effect or in any manner impairingthe dried web or its coating, and without limiting the length, width, ornumber of nozzles. The flow of impinged drying air along the surfaceacts as a curtain which isolates the surface from the spent air. Thearrangement To provide a multiplicity of nozzles with their respectivepressure gradients extending across the entire width of a web trainedover a' heated dmm results in holding the web more tightly to the drum,air and vapor being purged from the space between the web and the drum,

thereby increasing'heat fiow from the drum to the web. Also the masstransfer rate of vapor. away from the web is increased and the vaporpressure and the related saturation temperatureon the surface of the webare reduced, thereby increasing heat flow from thedrum to the web andmaking it possible to use higher drum temperatures without blowing offthe web. The vapor mass transfer coefficient is efficient.

There are essentially three primary features of the apparatus. They are:

proportional to the heat transfer e 1 In theidr awings':

by these opposing side walls and the interior spaces'of the respectivechannels open at' their ends to permit the escape of the vapor bearingair. In a preferred organization the air is recirculated through'afilter and heater with controlled continuous withdrawal of some of thesaturated air and its'replacement with unsaturated air. There is nothingfundamentally new about recirculation but, as will be shown hereafter,the drying results produced by the instant invention arevery. greatlysuperior to the drying achieved with any' other known device for thepurpose.

.FIG. lis a view taken in section of FIG. 2 through a typicalinstallation. Y v 1 FIG. 2 is 'a'view'inrear elevation of the deviceshown in FIG- 1, portions of the housing being broken away.

FIG.'3 is a greatly enlarged detail view of some of the nozzlestructure'shown in FIG. 1.

FIG. 4 is a view showing the drum in side elevation and the nozzlestructure in section on line 4-4 of FIG. 1, the'section also beingindicated at 4+4 in FIG. 3 to show that it passes th'roughone ofthenozzles.

FIG. 5 is a fragmentary detail view taken in section on the line 55 ofFIG. 3.

proaching closer than one-eighth of an inch, even if it .i

were practical to do so.

The nozzles cannot be brought so close to thework that contact mightoccur through ir= regularities in mechanical tolerances. Neither can thenozzles be withdrawn from the work substantially beyond one-half inchwithout largely destroying the advantage which this apparatus hasoverconventional dryers. Any

7 FIG. 6 is a fragmentary view in-perspective of the apparatus as itappears from the inner ends of the nozzles, portions of a roll and theweb to be dried being fragmentarily illustrated. a v V FIG. 7 is'a viewin perspective fragmentarily illustrating the ends of a bank ofnozzlesand the adjacent end wall of the plenum chamber all of the air,collecting chamber walls' being broken away.

loss of velocity at'the point of impingementofthe drying gas on thesurface of the work can be'compensated only i by increased power. I i

Whateverthe kinetic energy of the, air in any particular installation,thenoz'zle arrangement disclosed herein will produce atleastthirty'percent more heat transfer with e correspondingly increasedmass vapor transfer than can be achieved in any drying organizationwhich does not employ the arrangement'of the present invention.

Nozzle spacings up'to two inches are practical. How'- ever, there aremarked advantages if a spacing closely approximating one inch is usedwith the preferred area ratio of nozzle orifice to total area. Therelationship befer area 'o'f'thework upon which the jets from the nozzleis operating is veryimportant. As shown by the' charts included in thedrawings, the percent of orifice area should be'approximately threepercent of the heat tran sfer areaat all noz'zlespacings, the outsidelimits'b eing-a 7 minimum of;one ,anda half percent and a. maximum offive and a halfpercent with a preferred range of twofto four percent. IT The drying gas, herein called air for convenience, is sup- FIG. 8 is aview in perspective of a fragmentary portion of one of the individualtroughs that make up the nozzle bankp g i FIG. 9 is a diagrammatic viewon areduced scale showing the circulatory. system- I I FIG. 10 is a'viewin cross section showing a modified application of the invention to aweb passing over a fixed p ate.

, FIG. 11 isa diagrammatic view' in cross section through d a series ofdrying rolls to one of which a bank of drying nozzles embodying thepresent invention has been applied.

tween the'open area of the nozzle and the total heat trans:

FIG. 12 is a diagrammatic view showing the effect of the ets of dryinggases produced accordingto the invent10n.. V

FIGS. 1 3.and 1 4 are pressure charts showing the corn parative effect"of locating the. nozzles at different distances fromfthe work.

plied to the nozzles at such pressure as to'producevery hlgh Velocltles,a e fimg 09 to'be dried 'isrepresented by'a web 20 passing about adrying roll 2-1 which may be assumed to be the steam-heated feet'perminute. I M Sincethe'volum'e delivered from the'very narrow 'nozf-.

zles is small, the power requirementis not excessive and? the kineticenergy. expended" on the surface of the web is not destructive; Thesmall volume flow from an individual-nozzle has'little'energy in spiteofthe high'velocity of the jet? The net result-of these-factors is todisrupt the laminar boundary region with high frequency to promote highheat a and mass transfer rates. A convenient mechanical V 7 nection ofthe bases. of generallychannel-shaped members :with slightly flaredsides 'tothe otherwise'iopen side of a plenum chamber so that the basesof :the respective ,chane construction involves the coni 7 FIG. 15 isfachart. showing theefiect of the. distance of the nozzle from the surfaceof the work. FIG. 16 is a chart showing .the effect oftotal orificeareain relation to the total heat transfer meant the work. FIG. 17 is achart showing theeffect of distance between nozzles upon the heattransfer coefficient and upon the kinetic energy of air required toproduce a given rate of heat transfer; a i

In theembodiment shown in FIGS. to 9 the work roll of a conventionaldryer towhich the drying apparatus of the'present invention has'beenadded. As already noted, the inventionis not concerned either with themanels close the chamber except for nozzle openings between the opposingandoutwardly converging side "wallsof consecutive channels. Thusthenozzles are constituted terial of the web or. theliquidto be'vaporized.The invention is applic'able to. primary drying which involves theremoval from a freshly formed paper web of the liquid Qjjcornponentofthe furnish. 'It is alsoapplicable to machine felts and to crepe. Ithas been tested in thedrying of the printing done bya four-color rotaryprinting press. It is "also. usable tolp'reset aqueous coatings and tore i'move thevolatile, constituentsof, coatings; inks, and dyes.

The foregoing particulars 'are given byway of example and not by way oflimitation. 'Also, because air is the most readily availabledrying'medium, the invention will be described with. reference to airbut with the, underpartitions 33, 34, 35 as shown in FIG. 3.

standing that other drying gases are equivalent so far as the presentinvention is concerned.

For convenience, the entire apparatus is mounted within a hood or shroud25 bodily movable to and from the roll 21 which not only provides aplaten for the support of the web but also desirably contributes heat toexpedite the drying. One way of mounting the hood for advancing andretracting movement is to support it on the arms 26 from trunnions 27about which it may be withdrawn pivotally from the drum by cables 28 tofacilitate training the web about the drum, or to give access to thenozzles when cleaning is desired.

The rim 29 about the margin of the hood is in proximity to the peripheryof the drying cylinder 2]; but is usually spaced about one-half inchfrom the end of the cylinder and about one-half inch radially inwardlyfrom the surface of the cylinder. This rim reduces the area throughwhich fresh air is drawn into the system thereby providing moreeffective seal. it also serves to deflect the stream of air flowing outof return spaces between the nozzles hereinafter described.

Within the hood 25 is an inner wall 3% which provides a plenum chamber31 to which the drying air is admitted through pipe 32. The plenumchamber is shorter than the hood, its end walls 33, 34 being spaced fromthe ends of the hood. Support for the nozzles is provided by the walls33, 34 and by an intermediate partition or partitions 35 provided with asuificient number of apertures 36 to permit ready flow of air throughoutthe plenum chamber from one end to the other. The plenum chamber is sosized that the cross sectional area of flow at any point is considerablygreater than the total orifice area downstream of the point. Thisarrangement provides uniform distribution of air to all nozzle orificesif air is supplied under some pressure to the plenum. Since the orificesrepresent but a small open area the pressure plenum need not beexcessively large.

The internozzle channels, hereafter described, lead the saturated airfrom the surface of the web to the ends of hood 25, externally of walls33 and 34. From this external space the air is exhausted through a pipe38. Some of it is discharged through the blower 39 and replaced bymake-up air which leaks in to the extent that circulating air isdischarged. The rest passes through a filter 4t and heater 4i and isrecirculated by blower 42,

which delivers the air back into the pipe 32. Desirably, the return airtemperature may be about 300 F. and its pressure sufficient to develop aflow through the nozzles at the rate of about 16,000 f.p.m. This head isabout 16" water gauge presure at 70 F.

The two pipes may have beveled flanges at 43 which seat withcomplementary flanges of the circulatory pipes when the housing islowered to the operative position of 7 FIG. 1. The seated connection isshown in FIG. 4 and the flanges separately in FIG. 2.

'The inner wall of the plenum chamber 31 is composed entirely of theeighty-four channels 45 which in practice have been used to form therespective nozzles 50 and the vent spaces 51 as best shown in FIGS. 3, 6

I and 8. The number of channels is, of course, optional,

depending on the size of the bank of nozzles. The bases of therespective channels are welded to the bulkheads or As already indicated,the otherwise generally parallel side walls 53, 54 of the respectivechannels 45 flare slightly adjacent their ends and the proximate sideWalls of consecutive channels converge to form the nozzles 553. Whilenot critical, the included angle between the converging side walls whichform the nozzles 50 has been made about 20. It has been found that thechange in angle between the generally parallel portions of the sideWalls and the converging portions 53, 54' stiifens the nozzle and thereis less friction loss in the nozzle than would be the case if the sidewalls converged uniformly from the bases to the ends.

These nozzles are spaced from each other three-fourths of an inch to twoinches or desirably about one inch center to center. Importance ofnozzle spacing is clearly shown in FIG. 17 which demonstrates that thespacing is critical, the permissible range being three-fourths of aninch to two inches and the greatly preferred spacing being very close toone inch.

FIG. 17 shows the effect of distance between nozzles when the nozzlesare mounted one-fourth of an inch from the work surface, the orificeopenings aggregating 2.8 .ercent of the exposed surface and the airtemperature eing 400 F. The curve 77 shows the heat transfer coefficientat various nozzle spacings with air energy of 0.4 H.P./ft. The curve 78shows kinetic energy of air requ'red to maintain a heat transfercoefficient of 65 Btu/hr. ft. F. The preferred range of operation is atnozzle spacings between the broken lines indicated by the arrow 79. Thearea between nozzles can be kept small so that the velocity of theexhaust air is quite high, this being permissiblebecause the path offlow of the discharge air and vapor is isolated from the drying surfaceby the dynamic air curtain along the surface produced by the highvelocity nozzle discharge.

It has also been found practicable that each nozzle have an openingapproximately .023 inch wide and ten feet four inches long interruptedby spacers hereinafter referred to. However, while the dimensions arecritical within limits hereafter explained, the specific dimensions heregiven are merely illustrative.

The relationship between nozzle opening and nozzle spacing can beexpressed in terms of a ratio of nozzle opening to heat transfer area.The total orifice area should be not less than one and a half percent ormore than five and a half percent of the heat transfer area, thepreferred range being two to four percent for reasons clearly shown inFIG. 16.

In FIG. 16 the curve 71 shows the heat transfer coefficient in relationto the percentage of orifice area to heat transfer area when the nozzlesare spaced one inch apart, the air energy being constant at 0.4 H.P./Ft.The curve 72 is a similar curve showing the relationship when thenozzles are spaced one and a half inches apart. The curve 73 is based ona nozzle spacing of two inches and the curve 74 is based on a nozzlespacing of three inches from each other.

The preferred range of percentages lies between the broken verticallines indicated by the arrow 75 while the broken lines designated byarrow 76 show the outside limits within which practicable operationaccording to the invention is achieved.

As best shown in FIGS. 2 and 7 the respective channels 45 extend beyondthe respective plenum chamber end walls 34, 35. The spaces 55 leading tothe nozzles 50 are closed off outwardly of the end wall flange 56 byplates 57, 58 welded between the outer margins and ends of side wallportions 53, 54 of successive channels. To facilitate the escape ofsaturated air from the vent spaces 51, the bases of the channels are cutaway beyond the end wall flanges 56 as best shown in FIG. 7.

Each of the channels is provided at intervals along one of its flaredside Walls with the spacers 59 which are shown, by way of example,applied to the side wall 54 of the channel 45 separately illustrated inFIG. 8. It is practicable to use spacers 3 in width and 2 /2" center tocenter. When the several channels 45 are assembled in a bank, as shownin FIGS. 1, 3 and 6, their opposing side walls 53, 54 converge to form anarrow nozzle slot 50, the width of which is determined by the spacers59. The spacers should be present in the minimum number required tosupport the margins of side walls at the desired spacing. The spacersfacilitate assembly. They may comprise weld metal and later are securelyfused to the side walls at the tips of the nozzles by a spot weldingoperation, without, however, appreciably changing the desired orificeopening. The length ,known. 7 V V I The'removal of the nozzle beyondthree-fourths 'of .7 r of the spacers is so slightthat they do notmaterially affect operation. Thus the effective length of each nozzle isdesirably at least equal tothe width of the web'to be dried.

The desirable range of nozzle opening widths in relation to work areahas been found to be .015 to .055 In practice, these tend to lie withinthe range of .020 to .030. The opposed side walls 53 and 54 ofconsecutive channels constitute the nozzle and then converge with anangle up to 20 between them. Actually an angle of 3". or 4 issatisfactory. This angle is not critical but is largely a matter ofdesign, the 'jet expansion anglev being independent of nozzle design. rV

The nozzles 50 are located in very close proximity to the web. Thepreferred range of spacings is from oneeighth of an inch tothree-eighths of an inch, although spacing of one-eighth of an inch toone-half of an inch have been used successfully. FIG. 15 shows thatbeyond three-fourths of an inch there is no advantage over conventionaldryers.

FIG. 13 is a chart showing actual measurementf surface pressures exertedby a blast'of .airunder 12.2 water pressure issuing from a nozzle .015"in width spaced one-eighth of an inchfrom a surface 60. The

line 60 not. only represents the surface but represents the zero.pressure. b

As shown in FIG. 13, .the area-61 in which the" work surface 60 issubject to near-maximum pressure is only slightly in excess of thenozzle width and the pressure over this area is quite high, being nearlyequal to the stagnation point pressure at the apex of the curve. Fromthis maximumthe pressure falls off sharply to subatmospheric pressure inthe areas 62.at each side of the pres sure area. itresults in a very.steep pressure gradient along the The sharp reduction is importantbecause one-eighth of an inch to three-eighths of an inch as shown bythe vertical broken lines identified by arrows 83 and 84 in FIG. 15.Arrow'85'designates a vertical broken line representing a distance ofthree-quarters of an inch of travel of the jets from the'nozzles to thework, this being the top of what is regarded as a practicable range ofjet travel. FIG. 15 shows the effect 'of varying the distance betweenthe nozzles and the surface of the work with the air temperature andenergy constant. In making these curves, the air was held to atemperature of 400 F. and the air energy was held at 0.4 H.P./ft. Thecurve 80 a was taken with the nozzles of 0.028 inch orifice one inchapart. The curve 81 was made with the nozzles one and a half inchesapart and having openings 0.042 inch.

In the device of FIG..11 I have shown a whole series.

. In the construction of FIG. 10 the rolls 213, 214 are mere idlers andthe web 20 to be dried is fed over a supporting plate 70 with which thehood 251 is associated. Although the work-supporting platen constitutesthe stationary plate 70 instead of a roll such as those indicated ,at 21and 210, the arrangement of the nozzles 50 in a surface of the work inthe plane of the laminar boundary layer. This tends to subject themolecules constituting the laminar boundary layer to greater fluidshear, thereby increasing the rate of molecular flow in a direction suchas to makethe boundary layer thinner'and increase the rate of heatexchange and of drying. 7

With the nozzles spaced in'accordance with preferred practice, theresulting fiowis as shown in FIG. 12, the maximum continuum along thesurface between consecutive nozzles being no greater than halfthedistance between nozzles symmetrically in bothdirections fromthepoint 66 of impact to the intermediate areas of escape as indicatedby the arrows 67. This is important in view of the analogy hereinaftermade to the Pohlhausen theory of heat transmission through the boundarylayer and'because it means that every partof the work surface isconstantly beingexposed to fresh" and unsaturated drying air whereasprevious dryers have developed a pattern of flow in which the continuumis with the entire surface.

ence in pressure curve resulting from spacing the nozzle one-half inchfrom the work-surface instead .of onepractically coextensive r V i e W V55 Comparison of FIG. 13 with FIG. 14 shows'th'e diiferbank within hood251 is essentially the same operation is similar to that abovedescribed. 7

The principles underlying the invention are as follows: The problem isto remove a, vaporizable liquid from the surface of the work which, inthis instance, is represented by a moving web. The vaporizable'liquidmay be and the "present inIthe'web as a result of manufacture thereof orit may be present on the surface of the web as, for example, the resultof a coating or a printing operation. Regardless .of. the depth to.which the liquid penetrates the web, the' vapor mustalways be removedfrom the surface. f

. Wherever there is relative motion between a surface and a surroundinggas there exists the phenomenon known as a boundary layer resultingfrommolecular adhesion of molecules of gas to each other and to thesurface. In the case of'most dryers using air, the boundary layer isturbulent at its surface, and isv generally considered to consist ofthree" distinct layers or strata, the innermost, close to. the surface,of-the drum, being laminar. 1 Outside of the laminar stratum there is atransition or buffer straturn' in which the molecules are less closelybound and outside of this there is a turbulent outer'stratum wherein themolecules are bound to the drum but capable of turbulent movementrespecting each other. To remove the moisture from the web on thesurface ofthe drum, heat is caused to. pass inwardly through theboundary layer and the vapor must move eighth inch, everything elsebeingthe same. It will be noted from FIG. 14 that there is no longer any areaof sub-atmosphericpressure-such as th'at'shown at 62 in FIG. 13. Thearea-610 which is subject to super atmospheric pressure is much broadervdue to the expansion of the jet from thenozzjle. (Theex pansion angleis about 6 /2.) For'the same reason the pressure is also muchless.However, evaporation is still several in the use oftechniques previouslytimes as i great as aninch fromthe surface so far diffuses thejet'andreduces its pressure as to result in-a dryingloss so great innozzle ata distance in excess of three-fourths. of an inch from tliework See P161115 The "preferred rangeis outwardly through .that layer;Heat-and vapor transfer through the boundary 'layeris relatively slowdue to-the insulating effect of the gas.' It' is desirable to reducethethickness of the boundary layer and thereby its insulating effect.

In 'the-laminar inner'stratum' transfer takes place extremelyfislowly bymoleculardiffusion. 'Thermal gradicats are very steep. In' the turbulentstrata particles of fiuid do not move in orderly fashion but move ineddies.

This action causes rapid diffusion and therefore heat transfer rapidlyoccurs with relatively low gradients.

'The inner-laminar stratum presents the primary obstacle to the movementof both heat and vapor.

In the past, the effort has generally been'to supply substantial volumesof gas at 'relatively high velocity.

:However, in-such arrangements the velocity has to be limited because,at least in the, drying of paperor the like, there is a limit-to theamount of velocity and mass 59 which the paper can stand withoutdisplacement of fiber, and even disruption of the surface. By placingthe nozzles extremely close to the surface of the work, in accordancewith the arrangement previously described, I am able to use highervelocities and lower masses of air in a way to change thecharacteristics of the boundary layer to maintain a laminar layer whichis unusually thin because of its short continuum, whereby to bring abouttremendous increases in drying rate without endangering the surface ofthe work. However, if the nozzles are placed too close together or ifthe ratio of total nozzle area to treated work area is too large, theflow from adjacent jets mutually interferes so that a sudden loss indrying occurs.

In preferred practice the velocities at the nozzle orifice are in therange of 10,000 to 20,000 feet per minute. However, since the volume ofair from each nozzle is small, the amount of power required is notexcessive, and at a ratio of nozzle opening to surface area of two tofour percent the kinetic energy expended on the surface is within safelimits.

It is also a feature of the present invention that the nozzles are quiteclose together about the surface of the drum. This is true even thoughrelatively greater space is provided for the exhaust of air and vaporbetween nozzles than is possible in some of the prior art devices. Theadvantage of having the nozzles close together is that the morefrequently and thoroughly the boundary layer can be disrupted and itssaturated air withdrawn, the lower will be the Reynolds number and thegreater will be the evaporation rate.

Primary purpose of the nozzle arrangement is to provide high heat andmass transfer coefficients in the film of air on the surface of thematerial being dried. Since mass transfer in an air film or thecapability of the air film to allow evaporation is largely a function ofthe heat transfer coefficient, the latter will henceforth be used forsimplification but with the understanding that it is essentiallyidentified with the drying potential. Furthermore, since most air dryersare capable of being attached to an air circulation system which mayprovide air or other gas at various temperatures and humidities, thechief criterion of the efiiciency of an air dryer is the heat transfercoefiicient which depends to a large extent on the geometry of the givendryer.

As has previously been shown, the continuum of the boundary layer withrespect to the surface of the web consists of a length or distanceoriginating at a point on the surface directly under the nozzle which iscalled the stagnation point and continuing to a point on the surfacemidway between the nozzles. The boundary layer film developed under mynozzle arrangement is highly analogous to the proposition stated byPohlhausen in his development of a formula applicable to heat transferto or from a thin fiat plate immersed in a deep uniform fluid stream.(McAdams, Heat Transmission, 3rd edition, page 224.) The chiefdifference between the Pohlhausen proposition and the case of this dryeris that in the former a uniform flow exists at the origin ofthecontinuu-m and no pressure gradient exists in the direction of flow,whereas in this dryer a pressure potential exists at the origin which israpidly converted into high velocity flow with an accompanying steeppressure gradient in the direction of flow. Beyond the point wherepressure potential has been converted into high velocity flow and forthe greater part of the continuum the analogy is nearly exact.

. Little is known about the effect of a steep pressure gradient on theboundary layer.

If a pressure gradient is imposed on the boundary layer in the directionof flow, all particles in the film or boundary layer are subjected tothe pressure gradient in addition to the usual viscous drag and pull ofvertically" adjacent particles. Therefore, in the practice of thepresent invention, it is believed that there is a region of very highfluid shear close to the surface in the impingement area. This highshear is believed to be the cause of the very high heat transfer andmass transfer rates achieved by the present invention, as compared withthose predicted by the Pohlhausen formula cited in McAdams supra.

I have rearranged the Pohlhausen formula and grouped all of thequantities related only to the internal state of the fluid in a singleconstant It. The formula thus modified is as follows:

As in the Pohlhausen formula L represents the length measured from thesource of flow over which the heat transfer occurs. In my apparatus Lnecessarily is equal to one-half the distance between nozzles, since theflow under consideration is the fiow between the point of impingementagainst the work and the point at which the gases and vapor escape fromthe surface of the work (see FIG. 12).

In the Pohlhausen formula V is the stream velocity which is comparableto the impingement velocity in my apparatus. From this abridged formulait is apparent that a greater heat transfer coefficient, h will resultwith higher impingement velocity and/or shorter continuum length. In theactual measure of heat transfer, l have found that the length L has aneven greater effect than indicated by the modified formulaprobably due tthe high shear effect above referred to.

The impingement velocity is related to the velocity at the nozzleorifice, the size of the orifice, and the distance of the nozzle fromthe surface. The formula,

V D 1/2 V D relates these quantities according to my measurements. V/ Vis the ratio of impingement velocity to nozzle velocity with a limitingvalue of 1.0; D is the width of the nozzle orifice; Y is the distance ofthe nozzle from the surface. And 6 is a constant with a value of about.13 related to the expansion angle of the jet. It is evident from theformula that increasing the distance of the nozzle from the surface willdecrease the impingement velocity, and reducing the nozzle orifice willalso decrease the impingement velocity though not to the same degree.

FIG. 15 shows the effect of varying the distance of the nozzle from thesurface. As will be apparent, the heat transfer coefficient reaches amaximum at a point near one-eighth of an inch spacing of the nozzle fromthe surface. The chart shows that there is no material advantage inspacing the nozzle more than three-fourths of an inch from the work. Thespacing should preferably be kept lower than three-eightl1s of an inch.

The effect of varying the orifice opening in percentage ratio to heattransfer area is very clearly shown in FIG. 16 for different nozzlespacings. At all spacings the heat transfer coefficient falls off verysharply below one and a half percent and is seriously impaired abovefive and a half percent, the preferred percentage range being betweentwo and four percent.

It will be observed that in the above equation it 1s assumed that thequantity in parenthesis is less than unity. If it were above unity, theequation would cease to have significance from the standpoint of thepresent invention. Unity is virtually reached with a nozzle .032 inwidth and spaced from the Work by one-eighth of an inch. The formula isverified in FIG. 15. The invention contemplates as one of its requisitesthat a relatively high impingement velocity of the stream of dryinggases against the work, together with the relatively close spacingbetween such jets of drying gases, be used to reduce the continuum andminimize the thickness of the laminar portion of the boundary layerwhich is proximate the work. If the spacingof the nozzles from the workexceeds a value such that the value of the above equation exceedsunity,.no further advantage is gained.

Thus there is little advantage in spacing closer than about one-eighthof an inch. Ideally the value ,of unity for this quantity would bedesirable. a

Note further that it is desirable that the nozzle spacing from the Workbe several times the orifice width-in order' that maximum gas velocitywill be developed at the impingement point rather than between thenozzle edge a and the work. 7 I v As compared with competitive equipmentmy drying apparatus exhibits several advantages 1 not heretoforeregarded as possible of achievement. In addition to;

figures assume that the nozzles'will be of the transversely elongatetype herein shown. Y 7

My dryer has consistently effected removal-from 'a web surface ofvaporizable liquid at the, rate of eight pounds'to twenty pounds per,hour per square footof surface (depending on the application) by theuseof air only,'without any other source of heat.v One unit has removedsixty'pounds of water per hour per square thejnozzles'is quite uniformover the entire drying area. The flow from the nozzles. impinges on thesurface and is thusredirected to flow on the surface from the stagnationpoint opposite a nozzle to a point. midway'between nozzles where theair, leaves the surface. This flow of air at high energy on the surfaceinsulates the surface from the effect of spent air leaving the areathrough the internozzle channels. The surface is therefore contactedonly by the primary stream of air atiuniform-temperature and humidityfrom the nozzles. The critical test of the uniformity of drying for anyair dryer is operation at high temperature since, non-uniformity ofdrying is accentuated by high temperature. In practice my apparatus hasdried a web onehundredforty-eight inches wide with The character of thefoot of surface in combination with a heated cylinder and there appearsto be a" possibility .ofstill further improvement. a

an air temperature of approximately 680 Fahrenheit without measurablevariation. in dryness across the width of the'web leaving the dryer. 7

' V y device provides high drying capacityand uniform drying, and is atthe same time economical to operate. Although it is conventional touserelatively high pressure in the supply to :the drying unit, the flowthrough the nozzles is low; and at the specified ratios of nozzle areato work area, low fan'power is required (see FIG. 17).. In my apparatusthe drying potential varies approximately as the .24 power of the fanhorsepower in a given design 'of the device. Therefore excessive use ofpower produces little benefit; In practice my apparatus requires airdriving, power of approximately one-half horsepower per square foot ofdrying surface with an air temperature ;of 200 F. i

My apparatus lends itself to the conservation of heat in the dryingprocess} The first and last nozzles effectively seal the drying areafrom the surrounding atmosphere. In so doing half. of the flow from eachof these nozzles escapes to the atmosphere, but in a drying apparatussuch 'as mine with arelativ ely large number of nozzles, the

The previously demonstrated high drying capacity of my apparatus hasbeen successfully proven. In one case the addition of two of my devicesto a paper machine dryer section composed of twenty forty-eight inchcylinders increased the drying capacity of the machine inexcess of 30%.In another case a new design, of rotogravure press with greatly reducedlength of draw between decks :was-made possible by the use of my dryingdevice. In each of thesecases and in others the high drying rate in ashort space has beena decisive advantage.

percentage of the leakage is .very low. The remaining edges of thedrying area are sealed by maintaining close clearances between theexhaust curtain and the web support. Thus it'is' possible to recirculatea large percentage of the drying airwithout excessive loss. The air ex-7 hausted fromsuch a system contain's'large amounts of vapor which isconducive to high thermal efficiency.

7 Exhaust humidities in excess of .6'pound of water vapor The greatheatand mass transfer coefficients of my drying apparatus and method have,several incidental advantages. When my dryer is'used in conjunction withtheusual steam-filled cylinder of a conventional dryer, a large portionof the heat required for evaporation is supplied tothe web from thesurface-of'the cylinder. In some instances in the prior art thecylin'derisurface is web and the cylinder sufficient to force the webaway to' one pound of air'have been measured. Furthermore, the verycompactness of my dryer, doing a given drying job in a very small arealeads to high thermal efficiency since losses due to radiation andconvection are less per pound of waterevaporated;

In practice my dryer can operate with as little as ten percent of makeupair. Typical steam consumption in the use of my invention is 1.25 lbs.per. pound of evaporation. My device requires little space, has low heatloss,

,so hot as to develop high vapor pressure between the Q from thecylinder surface, thus sharply reducing. heat. transfer. Theuse of myinvention so reduces the'vapor:

' pressure on the outer surface of the web that the resulting gradientof vapor pressure through the web reduces the tendency forthe web to"blow off'the cylinder.' Moreover, the-increased rate of evaporationfrom the surface of the web sharply reduces'the temperature to which theweb is subjected andrnakes vit-p'ossible even to increase the amount ofheat-in theycylinder beyond the .point previously thought to be damagingto'the web.

web secured in my apparatus'is. an; important result of requires littlemaintenance, is easy'and safe to, operate,

hashigh production capacity,- and is inexpensive to pro- 1. Inaewebdryeri of type in'whi'ch the work to be dried is exposed to jets ofdrying gas, the combination with means for advancing .upon apredetermined path the work to be dried of a plurality of nozzlesextending continuously for substantially the fullwidth of the worktransversely of such path and 'very narrow ingthe direc- The uniformityof drying across'the entire width of a my nozzle arrangement. The reasonthat other attempts I to obtain high drying capacities have failed to beprac-. tical is because they were unable to dry uniformly across thewidth of webs commonly used by industry. In my.

app'aratus-theturbulent mixing or: air under pressure in tion ofmovement of the work, said nozzles being" directed toward the surface ofthe work to be dried at a distance of not more, than three-fourths of aninch therefrom and spaced not less than three-fourths inch and not morethan two inches from each, other, the ratioof total nozzle orifice areato total exposed work area being no less than one and'one-half pe'rcentand no morethan five and onehalf percent.

2. ,Thejdevice of claim 11 inwhichthe nozzles com- "prise elongated.o'pposing walls and means closing the nozzles at the, ends of said'walls,.lthe side margins of said walls. being angularly convergent toprovide nozzle slots greatly elongated ascOmparedwith their width, thesaid 13 opposing walls of each nozzle being spaced from the opposingwalls of adjacent nozzles to provide clearance for the Withdrawal ofgases from the surface of the work against which jets of drying gas aredirected through said nozzles.

3. In a web dryer of the type in which the work to be dried is exposedto jets of drying gas, means for jetting gas upon the work and includinga plurality of generally parallel nozzles elongated transversely of thework to be dried and directed toward the surface of the work to be driedand having spaces between them for the escape of gas which has actedupon such work, said nozzles being spaced from the work at a distance ofnot more than three-fourths of an inch therefrom and spaced not lessthan three-fourths of an inch nor more than two inches from each other,said nozzles having an aggregate open orifice area totalling between l/2% and 5 /2% of total heat transfer area.

4. In a web dryer of the type in which the work to be dried is exposedto jets of drying gas, means for jetting gas upon the work and includinga plurality of generally parallel nozzles elongated transversely of thework to be dried and directed toward the surface of the work to be driedand having spacings between them for the escape of gas which has actedupon such work, the ratio of the total orifice area to the total area ofthe work exposed to said jets being between two and four percent, thedistance between said nozzles approximating one inch between centers andthe distance from the nozzles to said surface being not more thanthree-eights of an inch.

5. In a dryer, the combination with a hood and means providing a plenumchamber within the hood, of a plurality of nozzles opening from theplenum chamber toward the surface of the work to be dried at a spacingnot to exceed three-fourths of an inch from such work and mutuallyspaced from each other at a spacing not to exceed approximately twoinches to provide inter-nozzle passages communicating with the hoodexternally of the plenum chamber, means for supplying a drying gas underpressure to said chamber for delivery at high velocity from saidnozzles, the width of each such nozzle being less than the spacingbetween the nozzle and the work, whereby the velocity of gas impingementon the work exceeds the velocity of gas movement along the work, and acirculatory system of which said means constitutes a part, said systemincluding a return conduit from said hood and a supply conduit to saidplenum chamber, the plenum chamber having a wall opposite the work to bedried which comprises a series of channels having base portionsconnected to the plenum chamber, said channels having opposing flanges,means connecting the flange of one channel to the flange of the next atthe ends of said flanges to constitute said nozzles between successiveflanges, the space within the channels constituting said return passagesaccommodating flow of gases from the work to the hood, the side marginsof the channel flanges 'being mutually divergent whereby the opposingwalls constituting respective nozzles are convergent at the tips of saidnozzles, the nozzle openings being greatly elongated as compared withtheir said width and being spaced from each other between three-fourthsinch and two-inches,

center to center, the ratio of total nozzle opening to total l 4 workarea being in the approximate range of 1.5 percent to 5.5 percent.

6. The device of claim 5 in which the ends of respective channels extendbeyond the plenum chamber, the said means connecting the ends of thechannel flanges being disposed externally of the plenum chamber and thebase portions of respective channels being relieved externally of theplenum chamber, whereby the interior portions of respective channels areopened into the hood.

7. In a web dryer of the type in which the work to be dried is exposedto jets of drying gas, the combination with means for advancing along apredetermined path the work to be dried of a plurality of jets elongatedtransversely of said path and continuous for substantially the fullwidth of the work, said jets being very narrow in the direction of thework and directed toward the surface of the work to be dried andoriginating at a distance of not more than three-fourths of an inchtherefrom and at a spacing of not less than three-fourths inch and notmore than two inches from each other, the ratio of the total crosssectional area of the jets at their origin to the total exposed workarea being no less than one and one-half percent and no more than fiveand one-half percent.

-8. In a web dryer of the type in which the work to be dried is exposedto jets of drying gas, the combination with means for advancing the workto be dried along a predetermined path of means immediately adjacentsaid path for establishing a plurality of jets directed toward thesurface of the work to be dried, said means comprising nozzle slotselongated transversely of the direction of advance of the work to bedried and continuous for substantially the width of such work and beingat a distance of not more than three-fourths of an inch from the worksurface and constituting means for spacing the jets not less thanthree-fourths of an inch and not more than two inches from each otherand for giving them a total area at said means of not less than one andone-half percent or more than five and one-half percent of the totalexposed work area.

References Cited by the Examiner UNITED STATES PATENTS Re. 24,144 4/56Dungler 34-l60 1,724,645 8/29 De Long 34-122 X 1,933,960 11/33 Brabaek34-122 2,306,019 12/42 Hanson 34-160 2,622,343 12/52 Metcalfe 34-1222,627,667 2/53 Gillis 34-37 X 2,828,552 4/58 Brendel 34-l22 X 2,928,1853/60 Drew 34-122 X 3,077,675 2/63 Dickens 34-122 FOREIGN PATENTS 309,6509/55 Switzerland. 727,058 3/55 Great Britain. 773,908 5/57 GreatBritain.

NORMAN YUDKOFF, Primary Examiner.

GEORGE D. MITCHELL, CHARLES OCONNELL,

BENJAMIN BENDETT, Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE ()F ,;CORRECTION Patent No.3,176,412

April 6, 1965 Thomas A. Gardner It is hereby ent requiring corrcertified that error appears in the above numbered patcorrected below.

ection and that the said Letters Patent should read as Column 10, lines11 to 13, shown below inst the formula should appear as ead of as in thepatent:

1/2 r m (T) same column 10, lines 33 to 37 the formula sh shown belowinstead of as in t ould appear. as he patent.

- l/2 V D v MP same column 10, line 42, for "6" read oz Signed andsealed this 28th day of September 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner :ofPatents

1. IN A WEB DRYER OF THE TYPE IN WHICH THE WORK TO BE DRIED IS EXPOSEDTO JETS OF DRYING GAS, THE COMBINATION WITH MEANS FOR ADVANCING UPON APREDETERMINED PATH THE WORK TO BE DRIED OF A PLURALITY OF NOZZLESEXTENDING CONTINUOUSLY FOR SUBSTANTIALLY THE FULL WIDTH OF THE WORKTRANSVERSELY OF SUCH PATH AND VERY NARROW IN THE DIRECTION OF MOVEMENTOF THE WORK, SAID NOZZLES BEING DIRECTED TOWARD THE SURFACE OF THE WORKTO BE DRIED AT A DISTANCE OF NOT MORE THAN THREE-FOURTHS OF AN INCHTHEREFROM AND SPACED NOT LESS THAN THREE-FOURTHS INCH AND NOT MORE THANTWO INCHES FROM EACH OTHER, THE RATIO OF TOTAL NOZZLE ORIFICE AREA TOTOTAL EXPOSED WORK AREA BEING NO LESS THAN ONE AND ONE-HALF PERCENT ANDNO MORE THAN FIVE AND ONEHALF PERCENT.